There are various known methods to switch electrical signals on and off. There is a mechanical switch, such as a light switch, in which a device is manually operated to connect or disconnect electricity from a source to a load. There are relays, which are electrically operated switches, such as those that use electromagnets, coils, springs, or mechanical contacts to mechanically operate a switch.
A solid state relay is an electronic switching device with no moving parts that relies on the electrical and optical properties of solid state semiconductors to perform switching functions. A typical solid state relay switches on and off when a small control voltage is applied to its control terminals. A solid state relay may be used to control alternating current (AC) or a direct current (DC) provided to a load. Furthermore, solid state relays offer greater performance and/or reliability than electromechanical switches or relays. In many cases, integrated-circuit based switches, such as silicon controlled rectifiers, TRIACs, gate turn-off thyristors, power transistors, and the like may be used as output switches within the solid state relay. In many cases, these IC-based switches may be controlled from an external control circuit, where the control signal may be isolated from the switch circuit by use of an optocoupler, a transformer, or other such optical device. This isolation may be used to reduce conducted electrical emissions from being introduced into the low-voltage DC control circuitry. However, due to the nature of semiconductor-based devices, (e.g., turn-on voltages, threshold voltages, etc.), the solid state switches may cause transient disturbances on an AC supply line. The transient disturbances may cause radio frequency (RF) disturbances that may be introduced onto the AC supply line (e.g., conducted emissions). In turn, these may interfere with the operation of other equipment using the same AC supply line.
To minimize the effect of the conducted emissions, one could install external filters. However, installing external filters may increase the size and/or cost of the equipment into which the solid state relay is installed.